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Controlled Attenuation Parameter (CAP) Explained

The Controlled Attenuation Parameter (CAP) is a non-invasive ultrasound-based method used to measure the fat content of the liver, often combined with liver elastography.

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Things worth knowing about "Controlled Attenuation Parameter"

The Controlled Attenuation Parameter (CAP) is a non-invasive ultrasound-based method used to measure the fat content of the liver, often combined with liver elastography.

What is the Controlled Attenuation Parameter?

The Controlled Attenuation Parameter (CAP) is a diagnostic tool used alongside transient elastography (e.g., with the FibroScan® device) to non-invasively quantify the degree of fat accumulation in the liver. CAP measures the attenuation of ultrasound waves as they pass through liver tissue: the higher the fat content, the greater the attenuation of the ultrasound signal.

The procedure is painless, radiation-free, and can be performed on an outpatient basis. It produces a value expressed in dB/m (decibels per meter), which directly correlates with the degree of hepatic steatosis (fatty liver disease).

Background and Clinical Relevance

Hepatic steatosis (fatty liver) is one of the most prevalent liver conditions worldwide. It can result from a variety of causes, including:

Non-alcoholic fatty liver disease (NAFLD/MASLD)
Alcohol-related fatty liver disease
Metabolic syndrome, type 2 diabetes, or obesity
Certain medications or genetic disorders

If left untreated, fatty liver can progress through inflammation (steatohepatitis) to liver fibrosis and ultimately cirrhosis. CAP enables early detection of liver fat accumulation before serious damage occurs.

How is CAP Measured?

CAP is measured simultaneously with transient elastography and takes only a few minutes. The patient lies on their back with one arm raised, and an ultrasound probe is placed over the right side of the ribcage. The probe emits ultrasound waves that travel through liver tissue, and the degree of signal attenuation is automatically calculated and reported as the CAP value.

Reference Values and Interpretation

CAP values are generally interpreted as follows (reference ranges may vary slightly by device and guideline):

Below 238 dB/m: No or minimal steatosis (< S1)
238–259 dB/m: Mild steatosis (S1, approximately 5–33% fat content)
260–291 dB/m: Moderate steatosis (S2, approximately 34–66% fat content)
292 dB/m and above: Severe steatosis (S3, above 66% fat content)

Advantages and Limitations

Advantages

Non-invasive: no liver biopsy required
Quick and painless to perform
Simultaneous assessment of liver stiffness (fibrosis) and fat content (steatosis)
Good reproducibility and cost-effective compared to biopsy

Limitations

Results may be less accurate in patients with severe obesity
Does not provide information on inflammatory activity or fibrosis stage alone
Possible variability depending on device and operator
Standardization of cut-off values is still evolving

Clinical Applications

CAP is primarily used in the following clinical settings:

Diagnosis and monitoring of non-alcoholic fatty liver disease (NAFLD/MASLD)
Screening in patients with metabolic syndrome, type 2 diabetes, or obesity
Assessment of treatment or dietary intervention outcomes
Pre-operative liver evaluation
Complementary diagnostics in chronic liver disease

References

Sasso M. et al. - Controlled attenuation parameter (CAP): a novel VCTE guided ultrasonic attenuation measurement for the evaluation of hepatic steatosis. Ultrasound in Medicine and Biology, 2010.
European Association for the Study of the Liver (EASL) - EASL Clinical Practice Guidelines on non-invasive tests for evaluation of liver disease severity and prognosis. Journal of Hepatology, 2021.
Ferraioli G. et al. - Liver ultrasound elastography: An update to the world federation for ultrasound in medicine and biology guidelines and recommendations. Ultrasound in Medicine and Biology, 2022.

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